Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the intake manifold of the engine, thereby increasing engine power.
The turbocharger shaft is supported by a central bearing housing disposed between the turbine and compressor wheels. The turbocharger shaft rotates within a bearing cartridge housed in an axial bore of the bearing housing.
The turbocharger shaft is conventionally supported for rotation in a bearing housing by a bearing assembly comprising inner and outer races radially separated by bearing elements, the bearing housing having a bore through which the shaft extends.
Lubricating fluid, in the form of oil, is fed to the bearing assembly under pressure from the oil system of the engine via an oil inlet, gallery and passages. Typically, an outer surface of the outer race has shallow annular recesses for receipt of the lubricating oil, which provide annular clearances between the inner surface of the bearing housing and the outer surface of the outer race. The recesses provide a thin film of oil between the outer race and the bearing housing. The film acts as a squeeze film damper and damps rotary motion and vibration caused by rotation of the shaft. From the recesses, the oil flows into the bearing assembly through inlet passages in the outer race. The oil then flows through the bearing assembly, to provide lubrication within the assembly, before passing out of the bearing assembly through an outlet in the outer race to an oil sump. It is important that the outlet is oriented correctly to ensure that the lubricating fluid within the bearing assembly is able to drain under the action of gravity to a lubricating fluid sump.
The inner race of the bearing assembly is fixed to the turbocharger shaft and rotates with the turbocharger shaft. It is necessary for the outer race to be fitted freely within the bearing housing, to allow for assembly, thermal expansion and the squeeze film damping. However, the outer race needs to be constrained to prevent both axial and rotational movement of the outer race. This is necessary to ensure that the outlet in the outer race is oriented correctly to ensure that the lubricating fluid within the bearing assembly is able to drain under the action of gravity to a lubricating fluid sump. This is also necessary to ensure that the inlet passages in the outer race are correctly oriented relative to the bearing housing, so as to allow oil to pass through the inlet passages into the bearing assembly. Correctly orienting the outer race of the bearing assembly, during assembly of the turbocharger, can be problematic. In addition, it can be difficult to maintain the outer race of the bearing assembly in the correct orientation, during operation of the turbocharger.
Typical current designs use a pin that passes from the bearing housing into the outer race, so as to rotationally fix the outer race relative to the bearing housing. However, such an arrangement can be difficult to assemble, requires multiple component parts and accordingly is relatively expensive.
Furthermore, providing an effective sealing system to prevent oil leakage from the central bearing housing into the compressor or turbine housing can be problematic. Oil leakage is regarded as a particular problem at the compressor end of the turbocharger since at low boost pressures (e.g. when the engine is idling or when the turbocharger is operated in engine braking mode) there can be a significant pressure drop from the bearing housing to the compressor housing which encourages oil leakage into the compressor housing. In order to counter such leakage it is known to include an oil slinger as part of a thrust bearing assembly at the compressor end of the bearing housing. An oil slinger is an annular component which rotates with the turbocharger shaft and has surfaces or passages arranged for propelling oil away from the shaft as it rotates, and in particular from the passage through the bearing housing into the compressor housing. Generally an annular splash chamber defined in the bearing housing collects the oil displaced by the slinger, provides for its recirculation and typically allows it to drain to a sump.
Whereas the turbine of a turbocharger drives a compressor, in a power turbine the end of the turbine shaft remote from the turbine wheel transmits power via a mechanical coupling. In a turbocompound engine two turbines are provided in series with a power turbine connected in series with the turbine of a turbocharger. The power turbine is used to generate additional power and a gear wheel may be fixed to the end of the power turbine shaft in order to transmit that power to the crankshaft of the engine via an appropriate coupling (such as for example a fluid coupling or a gear or other drive mechanism), hydraulically, mechanically or electrically. As with a turbocharger, the shaft of a power turbine is supported on bearing assemblies, including appropriate lubricating systems, located within a bearing housing connected to the turbine housing. The bearing arrangement at the turbine end of the shaft may be substantially the same as that found in a turbocharger, although the bearing arrangement at the drive end of the shaft may be a ball bearing assembly.